Since its inception in the early 1980's, transcranial ultrasound Doppler imaging has demonstrated an ability to measure blood flow, hemorrhages, and perfusion in the brain. Recent research has also examined the possibility of diagnosing certain degenerative disorders such as Parkinson's and depression. These approaches use transcranial Doppler sonography (TCD), or the related transcranial color coded sonography (TCCS) to record frequency shifts in the sent and backscattered signals. Signals are generally applied with a relatively low frequency (˜2 MHz) probe in order to penetrate the skull bone, and are often used in conjunction with a contrast agent. There seems to be little advantage to applying frequencies above 2 MHz, as the skull's increased attenuation at higher frequencies causes the bone to act as a low pass filter, returning only the lower spectral frequencies. Center frequencies at and below 1 MHz have also been examined, showing a stronger signal intensity, but with an expected reduced resolution.
The primary setbacks for transcranial procedures are strong attenuation and the distortion caused by irregularities in the skull's shape, density, and sound speed. These properties collectively contribute toward destroying an ultrasound focus and/or decreasing the ability to spatially register received diagnostic information.
Coherent noninvasive focusing of ultrasound through the human skull has been suggested for a number of therapeutic and diagnostic implications in the brain. For example, ultrasound has been considered as a tool for the transskull treatment of brain tumors, targeted drug delivery, improved thrombolytic stroke treatment, blood flow imaging, detecting internal bleeding, and tomographic brain imaging. Although the human skull has been the barrier to clinical realization of many of these applications, studies have demonstrated both minimally invasive and noninvasive aberration correction methods for transskull focusing. Minimally invasive approaches use receiving probes designed for catheter insertion into the brain to measure the amplitude and phase distortion caused by the skull and then correct the beam using an array of ultrasound transducers. Alternatively, a completely noninvasive approach uses X-ray computed tomography (CT) images to predict the longitudinal wave distortion caused by the skull. Noninvasive focusing with a therapeutic array has been demonstrated with a longitudinal wave propagation model, but the amplitude of the focus was observed to drop when the focus was directed close to the skull surface.
The assumption that the transcranial propagation is composed of mainly longitudinal modes is valid for small incident beam angles, but rapidly breaks down beyond approximately 25°, as the longitudinal wave approaches its critical angle. This is a plausible explanation for reduced amplitude using the longitudinal model—As the focus is directed toward the periphery of the brain, an increasing number array elements are oriented at higher incident angles to the skull.
Modeling of shear waves has been dismissed as being either of insignificant amplitude or, if significant, that the resulting beam would be incoherent and hard to predict. The absence of significant information on the skull bone's elastic wave speeds also has inhibited its consideration in modeling. Similar issues exist with respect to ultrasound propagation through other bony structures.